It has been hypothesized that cells delay anaphase onset, and thus exit from mitosis, when the mitotic spindle lacks a normal bipolar symmetry. To evaluate this hypothesis we used the BMIRR video-LM facility to film multinucleated PtK cells that formed multipolar spindles. An analysis of the data revealed that anaphase onset is delayed in monopolar spindles containing unattached kinetochores, but not in multipolar spindles which do not contain unattached kinetochores. This data indicates that the metaphase-anaphase cell cycle checkpoint does not monitor spindle geometry. It further reveals that monopolar spindles are checkpionted not because they lack the proper spindle geometry, but because they have unattached kinetochores that signal wait anaphase. This work has been asbtracted to the 1995ASCB meeting.